In the hardware package you should have received with your case, there are a number of sturdy mounting screws with cylindrical heads (instead of the normal oval ones). To fasten your storage devices, you attach four of these screws per unit, with two on each side; note that there are fine-threaded ones for optical and floppy drives, and coarse-threaded ones for hard drives. Once the screws are attached, remove the front case bezel to expose the fronts of the drive bays, guide the screws into the slots of the appropriate bay, and push the device in until you feel the back-left screw-head engage the plastic locking mechanism at the back of the drive cage.
To remove a drive: with the front case bezel removed, lift the plastic drive-locking mechanism on the side of the drive cage and slide out the drive.